Polyesters, such as polyethylene terephthalate (PET) are prepared industrially in a two stage process. The first stage in PET preparation involves the direct esterification of terephthalic acid with ethylene glycol, or alternatively transesterification of a C1-C4 dialkylterephthalate with ethylene glycol to form a low molecular weight precondensate. In a second stage, the precondensate is polycondensed to form high molecular weight polyethylene terephthalate. Both stages typically employ catalytic acceleration.
Depending on the end use of the polyester, a further solid state polymerization step (SSP) is employed to arrive at the desired viscosity or molecular weight build up. The polyesters according to this invention are subjected to solid state polymerization.
Numerous compounds have been proposed as esterification, transesterification or polycondensation catalysts. Choice of catalyst effects the color, strength and processing properties of the end product. Choice of catalyst effects for example the amount of aldehyde generation. Choice of catalyst also controls selectivity of the reaction and effects the amount of impurities formed such as diethylene glycol, cyclic oligomers and carboxylic acid end groups.
Choice of catalyst also effects the time required to achieve a desired viscosity or molecular weight build up in the solid state polymerization step. For instance, titanate catalyzed polyesters are known to exhibit relatively slow SSP rates as compared to for example antimony catalyzed polyesters. Nonetheless, titanate catalysts are valued by the polyester industry since they can provide for fast polycondensation rates at low levels. The value of certain catalysts in the industry would be enhanced if their shortcomings could be overcome.
JP2002293909 is aimed at a method for producing polyester.
U.S. Pat. No. 7,205,379 discloses a process for the preparation of a stabilized polyester that is low in the generation of aldehydes.
U.S. Pat. No. 5,981,690 teaches poly(alkylene arylates) which are prepared using an organic titanate-ligand catalyst solution containing organic silicates and/or zirconates and, preferably, certain phosphorus compounds.
U.S. Pat. No. 5,453,479 is aimed at novel polyesterification catalysts comprising a phosphorus component and a titanium component which are useful in preparing improved blends of polyester and polycarbonate resins.
GB 1338091 is aimed at the production of highly polymeric polyesters of aromatic dicarboxylic acids and dihydric alcohols.
U.S. published app. No. 200510239929 teaches a polyester that can be produced substantially without using an antimony compound as a polycondensation catalyst.
It has now been found that where certain metal catalysts are employed in the esterification or transesterification or polycondensation steps of preparing a polyester, that the presence of certain phosphinate compounds provides for higher molecular weight build up, or viscosity increase, during a subsequent SSP step. That is, the SSP rate is increased. The high viscosity polyester obtained also has high brightness and low yellow color and exhibits little aldehyde formation on melt processing.